In many branches of industry, products are stored or processed in containers, e.g. tanks, vessels or others. It has become a very important field of measurement technology to determine a level of a medium inside a container. Level measurements are valuable means in process control as well as in process automation.
One method of level measurement used today involves the use of short electromagnetic waves, in particular microwaves. Microwave level measurement devices transmit microwaves through an opening in the container toward the surface of the medium inside and receive echo waves of the transmitted microwaves. From the received echo waves an echo function representing echo amplitude as a function of the distance or time is formed for each measurement. A transit time of a microwave signal to travel from the device to the surface and of its echo to return to the device is determined based on the echo function. The distance between the device and the surface is then determined based on the transit time.
Various microwave level measuring techniques are known which permit short distances to be measured by means of reflected waves. The most frequently used systems are pulse radar and frequency-modulated continuous wave radar (FMCW-Radar).
Pulse radar level measurement devices periodically send short microwave pulses. The transit time between the transmission of the microwave pulse and the reception of its echo is measured and the level is determined based on the transit time.
FMCW radar level measurement devices transmit a continuous microwave signal, which is periodically linearly frequency modulated. The frequency of the received echo signal differs from the frequency of the transmitted signal by an amount, which depends on the transit time between the emission and the reception of the corresponding echo.
The microwave signals are transmitted and received via antennas. Commonly used antennas are for example horn antennas comprising a metal horn, rod antennas comprising a dielectric rod or planar antennas using microstrip line technology, as for example described in U.S. Pat. No.-B1 6,266,022.
In many branches of industry, containers may not be opened, when in use, due to hygienic or safety reasons. In the Pharmaceutical Industry it is for example essential to keep the containers sealed, in order to prevent any kind of contamination, e.g. bacteria, from entering. Even air can cause contamination of a medium inside the container.
On the other hand, there is a need in industry, to be able to service, recalibrate, repair or even replace microwave level measurement devices installed on the container.
Today microwave level measurement devices are usually mounted on a mounting section surrounding an opening of the container. Removal of the device leaves the interior of the container exposed to the environment. Correspondingly, contamination of a medium inside the container is possible. In order to prevent contamination, the container needs to be emptied before the device can be removed and the entire container needs to be cleaned and or sterilized after the device has been reinstalled on the container before the medium is reintroduced to the container. This is a costly and time intensive procedure. In some applications, for example in the bio-pharmaceutical industry, it is impossible to stop production processes, to allow for repair or replacement of microwave level measurement device. The reason for this is, that a sanitary seal would be broken. This would contaminate the product and could mean, that biological actions could be disturbed and weeks or even month of fermentation would need to be restartet.
In order to avoid these costs and the related loss of production time, there is a need for an assembly for microwave level measurement that allows removal of the microwave level measurement device, without opening the container.
In the past there have been applications for microwave level measurement devices, where engineers have physically separated microwave level measurement devices from the contents of the container, in order to protect the level measurement device from difficult measurement conditions, caused for example by high and/or constantly varying temperatures or pressures or by explosive, corrosive or toxic media. This problem is addressed in WO 95/12113.
In WO 95/12113 an arrangement for measurement of a level of a medium in a container is described, comprising a mounting section surrounding an opening of the container. A horn antenna of a microwave level measurement device is mounted on the mounting section. The mounting section described in the application is an annular flange, which is welded onto the tank. A window transparent for microwaves is foreseen, which is held inside a holding ring. The window is made of glass or ceramic. The holding ring is screwed onto the container such that the isolation window covers the opening. The holding ring is surrounded by the mounting section. The holding ring is fastened by screws, which are screwed into tapped holes in the container wall surrounding the opening.
Because of the rather complex design of the assembly described, the horn antenna and thus the microwave level measurement device connected to it cannot be mounted on standard mounting sections as they are generally foreseen on industrial sites. In many branches of Industry, users are very reluctant to use non-standard mountings. Any non-standard part is likely to increase the cost of operation. They make operation logistics and stockholding of spare parts more complex. In addition, it might become more difficult to obtain approval for operation by national authorities.
The company Thermo Measure Teck, based in 25555 North IH 35, Round Rock, Tex. 78664-2015 in the United States of America offers an arrangement for measurement of a level of a medium in a container on the world wide web, comprising:                a mounting section surrounding an opening of the container,        a tubular adapter comprising a flange located on a first end of the adapter                    which is mounted on the mounting section of the container by the flange,                        an isolation window transparent for microwaves                    sealing a passageway through the adapter,            comprising an outer rim, which is clamped between the mounting section and the flange, and                        a microwave level measurement device, comprising a wave guide and a horn antenna.        
The horn of the antenna is mounted on the adapter by a specially designed flange surrounding an end section of the horn, which is mounted on a second end of the adapter facing away from the container.
A gasket is foreseen on either side of the isolation window, which can be made out of various materials such as Polytetrafluorethylen (PTFE), Polypropylene etc.
Both arrangements described above allow for the antenna and thus for the microwave level measurement device mounted on the antenna to be removed without opening the container.
In both arrangements, horn antennas are foreseen which are mounted by specially designed connecting parts holding not only the horn but also the entire microwave level measurement device in place. Horns available vary in size and shape. Because of the complicated design, standard connection cannot be used for mounting the horn in these arrangements.